RFR: 8340395: GenShen: Remove unnecessary check on card barrier flag

[William Kemper]

PLABs will only be non-null in the generational mode, so we do not need to check the mode before checking if a PLAB is non-null.

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Commit messages:
 - Remove unnecessary mode check

Changes: https://git.openjdk.org/shenandoah/pull/501/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=501&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340395
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah/pull/501.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/501/head:pull/501

PR: https://git.openjdk.org/shenandoah/pull/501